We have already confidered relation, and under different views; but it was the relation of parts to parts, of the parts of an animal to other parts of the fame animal, or of another individual of the fame fpecies.

But the bodies of animals hold, in their conftitution and properties, a clofe and important relation to natures altogether external to their own; to inanimate fubftances, and to the fpecific qualities of thefe, e. g. they hold a ftrict relation to the ELEMENTS by which they are furrounded.

I. Can it be doubted, whether the wings of birds bear a relation to air, and the fins of fifh to water? They are inftruments of motion, feverally fuited to the properties of the medium in which the motion is to be performed: which properties are different. Was not this difference contemplated, when the inftruments were differently conftituted?

II. The ftructure of the animal ear depends for its ufe not fimply upon being furrounded by a fluid, but upon the fpecific nature of that fluid. Every fluid would not ferve: its particles muft repel one another; it muft form an elaftic medium: for it is by the fucceffive pulfes of fuch a medium, that the undulations excited by the founding body are carried to the organ; that a communication is formed between the object and the fenfe; which muft be done, before the internal machinery of the ear, fubtile as it is, can act at all.